Method and apparatus for cleaning flue gas

ABSTRACT

A method and apparatus for cleaning a flue-gas stream containing hydrogen chloride and sulfur dioxide are disclosed. The flue gas is treated in a wet cleaner (5), thereafter injected with lime (9) and filtered through a filter (13). The method and apparatus are distinguished by the fact that a pan of the flue-gas is bypassed the wet cleaner (5) through a conduit (15), to join the remainder of the flue-gas stream before the filter (13). In this way, hydrogen chloride in the bypassed stream is reacted with injected lime to form calcium chloride, which promotes the sulfur-dioxide separation upon the injection of lime.

This invention relates to a method and an apparatus for cleaning aflue-gas stream containing hydrogen chloride and sulphur dioxide.

Ever higher demands have been placed on flue-gas cleaning in recentyears, such that flue-gas cleaning today includes not only theseparation of dust, such as fly ash, by means of e.g. an electrostaticprecipitator, but also the separation of gaseous impurities, such assulphur dioxide and hydrogen chloride, found e.g. in the flue gasesgenerated in refuse incineration. Sulphur dioxide can be separated bywet cleaning, for instance in a scrubber where the flue-gas stream iscontacted with an alkaline aqueous solution that absorbs sulphurdioxide. Also hydrogen chloride can be removed by wet cleaning in ascrubber where the flue-gas stream is contacted with an aqueous solutionthat absorbs hydrogen chloride. The separation of hydrogen chloride andsulphur dioxide is often performed in a combined wet cleaner, in whichcase hydrogen chloride is absorbed in a first stage and sulphur dioxideis absorbed in a second stage. The flue gases leaving such a combinedwet cleaner are substantially rid of solid particles, hydrogen chlorideand sulphur dioxide and have hitherto been regarded as sufficientlyclean to be let out into the atmosphere. As environmental standards areraised, inter alia as regards the removal of nitrogen oxides (NOx), suchflue gases have to be further cleaned and any remaining residues ofheavy metals and sulphur dioxide should be removed to the greatestpossible extent. Thus, when removing nitrogen oxides catalytically fromflue-gases, it is a considerable advantage if the incoming flue gases atthe most contain 5 mg of sulphur dioxide and 0.1 mg of sulphur trioxideper Nm³.

In order to achieve such additional cleaning of the flue gases after theconventional wet cleaning, lime is injected into the flue gases,preferably in combination with active carbon, after the wet cleaning soas to react with any remaining impurities, whereupon the resultingproduct is collected on a filter. The lime injected into the flue-gasstream usually is in powder form but may also be a slurry of lime inwater. Such a lime slurry is finely divided when injected into theflue-gas stream, and the water in the slurry is evaporated whencontacted with the hot flue-gas stream. The resulting lime particlesreact with the impurities in the flue-gas stream, preferably sulphurdioxide, and are then collected on a filter. Use can be made of a bagfilter in which filter bags of textile material are arranged in a filterchamber. The injected lime, preferably combined with active carbon, thendeposit on the filter bags, and the cleaned flue gas leaves the bagfilter. The lime-containing deposit on the filter bags may react withfurther impurities in the flue gases, such as sulphur dioxide, beforebeing eventually removed from the filter bags and collected at thebottom of the filter chamber to be discharged as waste.

An instance of the above prior-art technique is described in WarnerBulletin, Feb. 1, 1993, "Retrofitting Waste Incineration Plant--BelowDetectability Limits".

Furthermore, it is known, e.g. from Hans T. Karlsson, Jonas Klingspor,Marira Linn e and Ingemar Bjerle, "Activated Wet-Dry Scrubbing of SO₂ ",Journal of the Air Pollution Control Association, Vol. 33, No. 1,January 1983, pp 23-28, that the separation of sulphur dioxide by meansof lime in a filter, as described above, is considerably promoted by thepresence of calcium chloride (CaCl₂). In the absence of calciumchloride, the flue gases have to be contacted with a greater amount oflime, i.e. more lime has to be injected, involving an increase in costs.Naturally, the increase in lime consumption means that the bag filteryields a greater amount of spent lime to be discharged as waste, whichis a disadvantage. In order to remedy this disadvantage, it is desirablethat the sulphur-dioxide separation by means of lime in the filter takesplace in the presence of calcium chloride.

The object of the present invention is to remedy the disadvantagementioned above while enabling sulphur-dioxide separation from fluegases by means of lime after wet cleaning to take place in the presenceof calcium chloride. The above disadvantage is remedied and theinventive object is attained by the fact that a partial stream of thehydrogen-chloride-containing flue gases is bypassed thehydrogen-chloride-separating wet cleaning, to join the flue gases afterthe wet cleaning but before the filtration of lime.

Further distinctive features of the invention appear from the appendedclaims.

It should here be observed that UK 2,078,702 discloses flue-gas cleaningby spray-drying absorption involving lime injection as well assubsequent filtration in a filter chamber, a part of the flue gas beingbypassed the spray-drying-absorption stage and conducted, together witha part of the flue gas that has been treated in thespray-drying-absorption stage, through a special section of the filterchamber. The rest of the flue gas that has been treated in thespray-drying-absorption stage is conducted through the remainder of thefilter chamber, and the two flow-gas streams join with each other afterthe filter chamber. The object of such bypassing is to reheat the fluegases.

BRIEF DESCRIPTION OF THE DRAWING

For clarifying purposes, the invention will now be described withreference to the accompanying drawing, which schematically illustrates apreferred embodiment of an apparatus according to the invention.

FIG. 1 illustrates the method and apparatus for cleaning flue gas usedin the examples.

Flue gases which, inter alia, contain hydrogen chloride and sulphurdioxide are generated in a furnace 1, e.g. for the incineration ofrefuse. Through a conduit 2, the flue gases are conducted to a dustseparator 3, e.g. an electrostatic precipitator, for the separation ofparticulate material, such as fly ash. Then, the flue gases areconducted through a conduit 4 to a wet cleaner 5 for the separation ofhydrogen chloride and preferably of sulphur dioxide as well. Thethus-cleaned flue gases leave the wet cleaner 5 through an outletconduit 6. When leaving, the flue gases mostly have a comparatively lowtemperature, such as 50°-80° C., and in order to avoid corrosion of thesucceeding equipment, heating means 7, such as a heat exchanger, arepreferably provided for raising the temperature of the departing fluegases to 80°-120° C. After being wet-cleaned and preferably heated aswell, the flue gases are conducted through a conduit 8 on to a station 9for lime injection. As used herein, the term "lime" is meant toencompass limestone (CaCO₃) as well as burnt lime (CaO) and slaked lime(Ca(OH)₂). Through a conduit 11, the lime is fed to the station 9 from asupply 10. Preferably, the lime is combined with active carbon from asupply 12. As mentioned earlier, the lime is injected into the fluegases in powder form or, if so desired, as an aqueous slurry. If so, thewater content of the slurry in relation to the temperature of the fluegases is such that the water of the lime slurry, which is finely dividedwhen the slurry is injected, is evaporated, leaving particles thataccompany the flue-gas stream and are collected on filter bags in a bagfilter 13. The lime particles and other solids are filtered off in thebag filter 13, and the flue gases then leave through a conduit 14.

In order to enhance the efficiency of the removal of sulphur dioxidewhen lime is injected, the reaction between sulphur dioxide and limeshould, as mentioned in the foregoing, take place in the presence ofcalcium chloride. To this end, a part of the flue-gas stream isconducted, via a conduit 15, from the inlet conduit 4 of the wetcleaner, past the wet cleaner and to the outlet conduit 8 of the wetcleaner at a site before the filter 13 and, in the preferred embodimentillustrated in the drawing, before the station 9 for lime injection aswell. Since a part of the flue-gas stream is bypassed the wet cleaner 5,this part will not be subjected to the separation of hydrogen chloridetaking place in the wet cleaner. As a result, the hydrogen chloridepresent in the bypassed flue-gas stream may react with the injected limeto form calcium chloride, which in turn will enhance the efficiency ofthe sulphur-dioxide separation.

In order to attain the aimed-at effect, it is sufficient to bypass onlya minor amount of the flue-gas stream, preferably about 1-8% by volumethereof, and most preferred about 2-5% by volume. In order to obtain asuitable ratio of bypassed flue gas to flue gas conducted through thewet cleaner 5, a regulating means 16 is preferably arranged in thebypass conduit 15. The regulating means 16 regulates the flow in thebypass conduit 15 according to the flue-gas flow in the inlet conduit 4and/or the hydrogen-chloride content of the gas in the inlet conduit 4.

In the embodiment described above and illustrated in the drawing, thebypassed flue-gas stream 15 joins the flue-gas stream that has passedthrough the wet cleaner 5 before the station 9 for lime injection.However, the bypassed flue-gas stream 15 may alternatively join theflue-gas stream that has passed through the wet cleaner 5 after thestation 9 for lime injection, provided that it does so before the filter13.

The invention will now illustrated with the aid of a few illustrativeExamples. The apparatus employed in the Examples is of the typedescribed above.

EXAMPLE 1

The incineration of household refuse generated flue gases whichcontained 900±300 mg of HCl and 250±150 mg of SO₂ per Nm³. Then, 95% byvolume of the flue gases was conducted through a wet scrubber whichseparated HCl but basically no SO₂. After the wet scrubber, the fluegases contained less than 5 mg of HCl and less than 250 mg of SO₂ perNm³. The remaining 5% by volume of the total flue-gas stream wasbypassed the wet scrubber and joined the flue gases leaving the wetscrubber before the station for lime injection. The flue gases conductedto the station for lime injection contained about 50 mg of HCl and about250 mg of SO₂ per Nm³. The lime was injected in an amount five times thestoichiometrically required amount. After the injection of lime, theflue gases were conducted to a bag filter, after which the flue gasescontained less than 2.5 mg of HCl and less than 25 mg of SO₂ per Nm³.This implies a 95% efficiency in the separation of HCl and a 90%efficiency in the separation of SO₂. The chlorine content of the dustcollected by the bag filter was estimated at about 2-3%.

EXAMPLE 2

The same flue gases as in Example 1 were conducted through a wetscrubber which removed not only HCl but also SO₂. After the wetscrubber, the flue gases contained less than 5 mg of HCl and less than50 mg of SO₂ per Nm³. About 2% by volume of the total flue-gas streamwas bypassed the wet scrubber and joined the flue gases from the wetscrubber before the station for lime injection. The flue gases conductedto the station for lime injection then contained about 20 mg of HCl andabout 50 mg of SO₂ per Nm³. The lime was injected in an amount ten timesthe stoichiometrically required amount. After the injection of lime, theflue gases were conducted to a bag filter for filtering off the injectedlime and any other solid impurities present. The flue gas leaving thebag filter was analysed for HCl and SO₂ and was found to contain lessthan 1 mg of HCl and less than 5 mg of SO₂ per Nm³, which implies anefficiency of more than 95% in the separation of HCl and an efficiencyof more than 90% in the separation of SO₂. The chlorine content of thedust collected by the bag filter was about 2%.

I claim:
 1. In a method for cleaning a flue-gas stream which containshydrogen chloride and sulphur dioxide and is treated in a wet cleaner(5), thereafter injected with lime (9) and filtered through a filter(13), wherein the improvement comprises bypassing a portion of the fluegas around the wet cleaner (5) so as to be forwarded to the limeinjection station (9) so that the hydrogen chloride within the flue gasreacts with the injected source of lime to result in the formation ofcalcium dichloride.
 2. A method as set forth in claim 1, wherein about1-8% by volume of the flue-gas stream is bypassed the wet cleaner (5).3. A method as set forth in claim 2, wherein about 2-5% by volume of theflue-gas stream is bypassed the wet cleaner (5).
 4. A method as setforth in claim 1, wherein the flue-gas stream leaving the wet cleaner(5) is heated to a temperature of about 80°-120° C. before joining thebypassed flue-gas stream.
 5. A method as set forth in claim 1, whereinthe source of lime is injected into the flue-gas stream in powder form.6. A method as set forth in claim 1, wherein the injected lime is slakedsource of lime (Ca(OH)₂), burnt lime (CaO) or calcium carbonate (CaCO₃).7. In an apparatus for cleaning a flue-gas stream which containshydrogen chloride and sulphur dioxide, the apparatus comprising an inletconduit (4) for supplying a flue-gas stream to a wet cleaner (5), anoutlet conduit (6) for discharging a flue-gas stream from the wetcleaner (5), a station (9) for injecting lime into the flue-gas streamafter the wet cleaner (5), and a filter (13) for filtering off lime,wherein the improvement comprises a bypass conduit (15) which extendsfrom the inlet conduit (4) of the wet cleaner (5) to the outlet conduit(8) of the wet cleaner (5) before the filter (13) and through which apart of the flue-gas stream is bypassed the wet cleaner (5).
 8. Anapparatus as set forth in claim 7, wherein the bypass conduit (15) hasmeans (16) for regulating the gas flow.
 9. An apparatus as set forth inclaim 7 wherein the means (16) in the bypass conduit for regulating theflue-gas flow are adapted to do so according to the flue-gas flow in theinlet conduit (4) and/or the hydrogen-chloride content of the gas in theinlet conduit (4).
 10. An apparatus as set forth in claim 7, wherein theoutlet conduit (6) of the wet cleaner (5) is provided with heating means(7).
 11. A method as set forth in claim 2, wherein the flue-gas streamleaving the wet cleaner (5) is heated to a temperature of about 80°-120°C. before joining the bypassed flue-gas stream.
 12. A method as setforth in claim 3, wherein the flue-gas stream leaving the wet cleaner(5) is heated to a temperature of about 80°-120° C. before joining thebypassed flue-gas stream.
 13. A method as set forth in claim 2, whereinthe source of lime is injected into the flue-gas stream in powder form.14. A method as set forth in claim 3, wherein the source of lime isinjected into the flue-gas stream in powder form.
 15. A method as setforth in claim 4, wherein the source of lime is injected into theflue-gas stream in powder form.
 16. A method as set forth in claim 2,wherein the injected source of lime is slaked lime (Ca(OH)₂), burntsource of lime (CaO) or calcium carbonate (CaCO₃).
 17. A method as setforth in claim 3, wherein the injected source of lime is slaked lime(Ca(OH)₂), burnt lime (CaO) or calcium carbonate (CaCO₃).
 18. Anapparatus as set forth in claim 8, wherein the means (16) in the bypassconduit for regulating the flue-gas flow are adapted to do so accordingto the flue-gas flow in the inlet conduit (4) and/or thehydrogen-chloride content of the gas in the inlet conduit (4).
 19. Anapparatus according to claim 8, wherein the outlet conduit (6) of thewet cleaner (5) is provided with heating means (7).
 20. An apparatusaccording to claim 8, wherein the outlet conduit (6) of the wet cleaner(5) is provided with heating means (7).